You've never had to deal with someone who came to work while on drugs, have you?
Someone can go from comparatively normal to crazy enough to freak everyone out and maybe get someone hurt in no time flat. That's a bit of a problem, especially given that some people work at places where accidents like slitting one's wrist wide open, crushing one's hand and being killed by an item dropped by a forklift happen a little too regularly, even to people who are not thus impaired.
I think the GGP was referring mainly to workplaces that don't involve dealing with dangerous equipment. Besides there's a big difference between smoking some pot outside of work hours and coming to work under the influence of meth.
When I worked in low paid (mainly call centre) jobs, people coming to work high was a pretty common thing. The jobs were easy and repetitive enough that it didn't affect performance at all. In fact if they had started drug testing they would probably have lost many of their best employees.
Someone can go from comparatively normal to crazy enough to freak everyone out and maybe get someone hurt in no time flat. That's a bit of a problem, especially given that some people work at places where accidents like slitting one's wrist wide open, crushing one's hand and being killed by an item dropped by a forklift happen a little too regularly, even to people who are not thus impaired.